Cellular restriction of HIV infection in host cells reflects innate antiviral immunity. Monocytes are important immune sentinels and precursors of antigen presenting cells. Undifferentiated human monocytes are resistant to HIV infection despite the expression of HIV receptors. The mechanisms of postentry HIV restriction in monocytes are not fully understood. Given that monocyte-differentiated macrophages are permissive for productive HIV infection, it has been speculated that differentiation-dependent cellular cofactors in monocytes might be responsible for the anti-HIV activity. However, specific HIV restriction factors in monocytes remain to be identified. Our long-term goal is to elucidate the molecular mechanisms underlying the postentry HIV restriction in monocytes and to identify host factors that contribute to the restriction. Our preliminary studies indicate that i) HIV infection is profoundly blocked in monocytes freshly isolated from healthy donors despite efficient viral reverse transcription, while the nuclear import of HIV DNA is significantly impaired in HIV-infected monocytes;ii) The postentry HIV restriction does not correlate with the expression levels of a few HIV restriction factors reported in monocytes. Thus, we hypothesize that unidentified cellular factors in monocytes may account for the postentry restriction of HIV infection. Two specific aims are proposed to test this hypothesis. 1). To explore the HIV restriction phenotype by cell fusion between primary monocytes and HIV-permissive CD4+ T cells. 2). To perform a genetic screen for potential HIV restriction factors from a cDNA library of undifferentiated monocytes. The research design and methods include cell-fusion-based HIV infection assays and a genetic analysis for potential HIV restriction factors in monocytes. The proposed studies will provide new insights into understanding of HIV interactions with monocytes. PUBLIC HEALTH RELEVANCE: HIV infection is the leading killer worldwide among infectious diseases, incurring 2-3 million AIDS deaths annually. This proposal attempts to explore the mechanisms of HIV restriction in monocytes, which will help to understand host factor-HIV interactions and potentially aid in developing effective strategies to combat HIV infection.